This invention relates to a transistor switch circuit. More particularly, it relates to an automatic back-up circuit for memory or CPU circuits using the transistor switch circuit.
The memory circuit or CPU circuit usually needs to be driven by the back-up current for holding memory contents therein after an operating power supply for the main circuitry including such memory circuit was made off. In general, a back-up power supply in the form of a battery is prepared so that it can supply the back-up current at the same time of the switching off of the main circuitry power supply. Furthermore, a diode is connected so as to stop the drawing of the back-up current into the other circuits of the main circuitry. This diode conducts the operating current from the main power supply to the memory IC.
In such prior art back-up circuit arrangement, such unidirectional diode provides considerable voltage drop (it is about 0.6 V with a silicon diode and 0.25 V with a germanium diode). Therefor, with respect to the power supply voltage, it needs to pre-consider a voltage corresponding to this voltage drop. Also, from the viewpoint of low power consumption, such circuit using the diode is not favorable.
Accordingly, the object of the invention is to provide a transistor switch circuit for use with the automatic back-up circuit wherein in place of the unidirectional diode a switching mode transistor is effectively used so that the voltage drop across the transistor becomes very low (under 0.1 V).
The other objects and advantages of the invention will be apparent from the description relating to the illustrated embodiment of the invention.